Liquid crystal display device using OCB cell and driving method thereof

ABSTRACT

A liquid crystal display device capable of suppressing the occurrence of a back transition in OCB cells and displaying excellent images as well as a driving method thereof are provided. One frame period has a first period P 1  for writing a signal for initializing the state of a liquid crystal in pixel cells and a second period for writing pixel data in correspondence with an image signal in pixel cells, and a voltage level to be applied to each pixel cell is set in the first period such that each pixel cell retains a voltage Vsup higher than that in the second period.

TECHNICAL FIELD

The present invention relates to a method of driving an active matrixliquid crystal display device and a liquid crystal display device, andparticularly to a method of driving a liquid crystal display deviceusing an OCB (optically compensated birefringence) liquid crystal modewhich realizes a wide view-field angle and high-speed response and aliquid crystal display device.

BACKGROUND ART

As is widely known, a liquid crystal display device is used in largenumbers as a visual display device of a computer or the like and isexpected to be used more widely also for use in television in future.However, a TN type cell that at present is widely used has seriousproblems with its display performance when used as television, forexample, a narrow view-field angle, an insufficient response speed,deterioration in parallactic contrast, and blurring of moving images.

In recent years, studies on an OCB cell that is to be used instead ofsuch a TN type cell have been made. The OCB cell is characterized byhaving a wide view-field angle and a high-speed response compared to theTN type cell, so that the OCB cell can be regarded as a liquid crystalcell that is more suitable for displaying natural moving images.

In the following, a conventional method of driving a liquid crystaldisplay device and a liquid crystal display device will be described.

FIG. 14 is a block diagram showing the configuration of a conventionalliquid crystal display device.

In FIG. 14, X1, X2, . . . , Xn are gate lines, Y1, Y2, . . . , Ym aresource lines, 126 is a thin-film transistor (hereinafter referred to asa TFT) as a switching element, and a drain electrode of each TFT isconnected to a pixel electrode inside a pixel 106. Each pixel 106includes a pixel electrode, an common electrode that is a transparentelectrode and a liquid crystal interposed between these two electrodes.The common electrode is driven by a voltage (Vcom) supplied from ancommon driving part 105. The voltage Vcom supplied to the commonelectrode has two kinds of voltages, including a first reference voltageVref1 and a second reference voltage Vref2, and the voltage is suppliedwhile switching between them for each horizontal period.

103 is an IC (hereinafter referred to as a source driver) that outputs avoltage to Y1, Y2, . . . , Ym to be supplied to the pixel 106. 104 is agate driver for applying a voltage rendering the TFT 126 to be in an ONstate or a voltage rendering the TFT 126 to be in an OFF state. The gatedriver 104 applies an ON voltage sequentially to the gate lines X1, X2,. . . , Xn in synchronization with the supply of data to the sourcelines Y1, Y2, . . . , Ym by the source driver 103. A phase of thevoltage supplied from the source driver 103 is opposite relative to aphase of the voltage Vcom supplied to the common electrode. Thisdifference in voltage between the voltage Vcom supplied to the commonelectrode and the voltage applied to each pixel 106 via the source linesY1, Y2, . . . , Ym is a voltage that is applied to both ends of theliquid crystal inside the pixel 106, and this voltage determines thetransmittance of the pixel 106.

In addition, FIG. 15 is a diagram showing waveforms of the voltage Vcomsupplied to the common electrode, a source signal Vs serving as an imagesignal (VI) supplied to the source driver 103, gate signals Vg(n−1), Vg(n), Vg (n+1) applied to (n−1) line, n line, (n+1) line respectively andthe timing relationship thereof.

Such a driving method is the same when using the OCB cell as well aswhen using the TN-type cell. However, the OCB cell needs to be driven ina special manner at an operation stage of starting an image display,which is not required for the TN type cell.

As shown in FIG. 16, an OCB cell has bend configuration (white display)corresponding to a state capable of an image display (FIG. 16B), bendconfiguration (black display) (FIG. 16C) and splay configurationcorresponding to a state incapable of displaying (FIG. 16A). In order toshift the configuration from this splay configuration state to a bendconfiguration state (hereinafter referred to as a transition), specialdriving needs to be done such as application of a high voltage for afixed period of time. However, the driving concerning this transition isnot directly related to the present invention, so that it will not befurther explained.

However, the problem with this OCB cell was that even if transition tobend configuration is once effected by the aforementioned specialdriving, when a condition continues to proceed in which a voltage of apredetermined level or higher is not applied for longer than a fixedperiod of time, the OCB cell cannot maintain the bend configuration andthus returns to the splay configuration (hereinafter, this phenomenon isreferred to as a back transition).

DISCLOSURE OF THE INVENTION

Therefore, it is an object of the present invention to provide a methodof driving a liquid crystal display device that can suppress anoccurrence of a back transition and can display excellent images usingan OCB cell and a liquid crystal display device.

To achieve the aforementioned object, a driving method of a liquidcrystal display device according to the present invention is a methodfor driving a liquid crystal display device having a liquid crystalpanel that includes a plurality of source lines to which pixel data aresupplied, a plurality of gate lines to which scanning signals aresupplied, pixel cells positioned in matrix form in correspondence withintersecting points of the source lines and the gate lines, a sourcedriver that drives the source lines based on an input image signal, agate driver that drives the gate lines, and a back light, and the pixelcells are OCB cells. The driving method is characterized in that a firstperiod for writing a signal for initializing a state of a liquid crystalin the pixel cells and a second period for writing pixel data incorrespondence with the image signal in the pixel cells are providedselectively in one frame period, and a voltage level to be applied toeach pixel cell in the first period is set such that each pixel cellretains a voltage Vsup higher than that in the second period. Here, “asignal for initializing a state of a liquid crystal” refers to a signalto be written in the liquid crystal for rendering the OCB liquid crystalthat tries to return to the splay state more likely to remain in thebend state.

In the driving method according to the present invention, it ispreferable that a ratio occupied by the first period in one frame periodis set to be less than 20%.

Furthermore, it is preferable that when a voltage of a predeterminedlevel or lower is applied to the pixel cell, it is judged that the firstperiod needs to be set in a next frame, and the first period is set inthe next frame.

In addition, it is preferable that when a voltage of a predeterminedlevel or lower is applied to the same pixel cell continuously in apredetermined number of preceding frames including a current frame, itis judged that the first period needs to be set in a next frame, and thefirst period is set in the next frame.

Moreover, it is preferable that the voltage Vsup is set variably foreach frame.

In this case, it is preferable that when it is judged that the firstperiod needs to be set, a voltage Vsup to be applied in a next frame isset to be at a level not less than a voltage Vsup applied in animmediately preceding frame, while when it is judged that the firstperiod does not need to be set, a voltage Vsup to be applied in a nextframe is set to be of a level not more than a voltage Vsup applied in animmediately preceding frame.

Alternatively, it is preferable that a length of the first period is setvariably for each frame.

In this case, it is preferable that when it is judged that the firstperiod needs to be set, a first period to be set in a next frame is setto be not less than a length of a first period set in an immediatelypreceding frame, while when it is judged that the first period does notneed to be set, a first period to be set in a next frame is set to benot more than a length of a first period set in an immediately precedingframe.

Furthermore, it is preferable that the back light is controlled by usingback light luminance control means that controls the brightness of theback light such that the back light lights up brighter in the frame inwhich the first period is set than in the frame in which the firstperiod is not set.

Furthermore, it is preferable that the back light is controlled by usingback light luminance control means that controls the brightness of theback light such that the back light lights up bright in correspondencewith a length of the first period.

Furthermore, it is preferable that a length of the first period iscontrolled by a result of calculating an average luminance level by animage signal input in a predetermined number of preceding frames and anaverage luminance level by an image signal to be input in a currentframe.

In this case, it is preferable that when a difference between an averageluminance level by an image signal input in a predetermined number ofpreceding frames and an average luminance level by an image signal to beinput in a current frame is larger than a predetermined level, the firstperiod is set to a predetermined length in a next frame.

Moreover, in the driving method according to the present invention, itis preferable that it is detected whether an input image signal is amoving image or a static image, and as a result of detection, the firstperiod is set longer than a predetermined length when it is judged thatthe input image signal is a moving image, and the first period is setshorter than a predetermined length when it is judged that the inputimage signal is a static image.

Moreover, in the driving method according to the present invention, itis preferable that when the image signal as a digital signal isconverted to an analog signal inside the source driver, a referencevoltage used for conversion is switched in synchronization with adriving timing of the source line and the gate line.

Moreover, in the driving method according to the present invention, itis preferable that the pixel data are supplied to the source lines innot more than half a time that can be spent for scanning one scanningline in one frame.

Alternatively, it is preferable that a voltage corresponding to pixeldata for one screen is applied to each pixel cell for not more than halfa time of one frame period.

To achieve the aforementioned object, a liquid crystal display deviceaccording to the present invention is a liquid crystal display devicehaving a liquid crystal panel that includes a plurality of source linesto which pixel data are supplied, a plurality of gate lines to whichscanning signals are supplied, pixel cells positioned in matrix form incorrespondence with intersecting points of the source lines and the gatelines, a source driver that drives the source lines based on an inputimage signal, a gate driver that drives the gate lines, and a backlight, and the pixel cells are OCB cells. The liquid crystal displaydevice is characterized in that a first period for writing a signal forinitializing a state of a liquid crystal in the pixel cells and a secondperiod for writing pixel data in correspondence with the image signal inthe pixel cells are provided selectively in one frame period, and meansfor setting a voltage level to be applied to each pixel cell in thefirst period such that each pixel cell retains a voltage Vsup higherthan that in the second period (driving control part) is provided.

In the liquid crystal display device according to the present invention,it is preferable that the setting means (the driving control part) setsthe voltage Vsup variably for each frame.

Alternatively, it is preferable that the setting means (the drivingcontrol part) sets a length of the first period variably for each frame.

Furthermore, it is preferable that the liquid crystal display deviceaccording to the present invention further includes back light luminancecontrol means (back light control part) for controlling the brightnessof the back light, wherein the back light luminance control meanscontrols the back light such that the back light lights up bright incorrespondence with a length of the first period.

According to the aforementioned method and the configuration, it ispossible to suppress an occurrence of a back transition and to easilyset the shortest Vsup hold period and the minimum Vsup voltage capableof suppressing a back transition. Therefore, it becomes possible todisplay excellent images by reducing the effects of the deterioration indisplay luminance as much as possible by inserting the Vsup hold period.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 1 of the present invention.

FIG. 2 is a timing chart for an common voltage Vcom, a source signal Vsand a gate signal Vg for driving the liquid crystal display device shownin FIG. 1 with respect to a given input image signal.

FIG. 3 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 2 of the present invention.

FIG. 4A is a timing chart for a vertical synchronous signal afterdouble-speed conversion is performed for driving the liquid crystaldisplay device shown in FIG. 3 with respect to a given input imagesignal.

FIG. 4B is a timing chart for an image signal (Vs) after double-speedconversion is performed for driving the liquid crystal display deviceshown in FIG. 3 with respect to a given input image signal.

FIG. 4C is a timing chart for a signal level detection signal (DS) fordriving the liquid crystal display device shown in FIG. 3 with respectto a given input image signal.

FIG. 5 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 3 of the present invention

FIG. 6A is a timing chart for a vertical synchronous signal afterdouble-speed conversion is performed for driving the liquid crystaldisplay device shown in FIG. 5 with respect to a given input imagesignal.

FIG. 6B is a timing chart for an image signal (Vs) after double-speedconversion is performed for driving the liquid crystal display deviceshown in FIG. 5 with respect to a given input image signal.

FIG. 6C is a timing chart for a Vsup period provision signal (VsupPS)for driving the liquid crystal display device shown in FIG. 5 withrespect to a given input image signal.

FIG. 7 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 4 of the present invention.

FIG. 8A is a timing chart for a vertical synchronous signal afterdouble-speed conversion is performed for driving the liquid crystaldisplay device shown in FIG. 7 with respect to a given input imagesignal.

FIG. 8B is a timing chart for an image signal (Vs) after double-speedconversion is performed for driving the liquid crystal display deviceshown in FIG. 7 with respect to a given input image signal.

FIG. 8C is a timing chart for a Vsup period provision signal (VsupPS)for driving the liquid crystal display device shown in FIG. 7 withrespect to a given input image signal.

FIG. 8D is a timing chart for a back light luminance control signal BC′for driving the liquid crystal display device shown in FIG. 7 withrespect to a given input image signal.

FIG. 9 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 5 of the present invention.

FIG. 10 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 6 of the present invention.

FIG. 11 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 7 of the present invention.

FIG. 12A is a diagram showing input-output characteristics of a sourcedriver shown in FIG. 11 resulting from switching to a reference voltageVREF1.

FIG. 12B is a diagram showing input-output characteristics of a sourcedriver shown in FIG. 11 resulting from switching to a reference voltageVREF2.

FIG. 12C is a diagram showing input-output characteristics of a sourcedriver shown in FIG. 11 resulting from switching to a reference voltageVREF3.

FIG. 12D is a diagram showing input-output characteristics of a sourcedriver shown in FIG. 11 resulting from switching to a reference voltageVREF4.

FIG. 13 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 8 of the present invention.

FIG. 14 is a block diagram showing the configuration of a conventionalliquid crystal display device.

FIG. 15 is a timing chart for an common voltage Vcom, a source signal Vsand a gate signal Vg for driving the liquid crystal display device shownin FIG. 14 with respect to a given input image signal.

FIG. 16A is a schematic view showing an OCB cell in a splayconfiguration state.

FIG. 16B is a schematic view showing an OCB cell in a bend configuration(white display) state.

FIG. 16C is a schematic view showing an OCB cell is in a bendconfiguration (black display) state.

FIG. 17 is a graph showing an applied voltage-transmittance curve of ageneral OCB cell.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, preferable embodiments of the present invention willbe described with reference to the drawings.

Embodiment 1

FIG. 1 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 1 of the present invention. InFIG. 1, the liquid crystal display device includes a line double-speedconversion part 101 to which an image signal (VI) and a synchronoussignal (SYNC) are input, a driving control part 102, a source driver103, a gate driver 104, an common driving part 105 to which first tofourth reference voltages Vref1 to Vref4 are input and from which ancommon voltage Vcom is output, a pixel cell 106, a back light controlpart 107 to which a back light luminance control signal (BC) is input,and a back light 108. Furthermore, the pixel cell 106 includes a liquidcrystal 116 and a switching element (TFT) 126.

In the following, a method of driving a liquid crystal display deviceaccording to Embodiment 1 of the present invention will be described byreferring further to FIG. 2.

FIG. 2 is a timing chart for an common voltage Vcom, a source signal Vsand a gate signal Vg for driving the liquid crystal display device shownin FIG. 1 with respect to a given input image signal.

In FIG. 2, the common voltage Vcom is provided from a drain electrode(D) of the TFT 126 formed on a cell to an common electrode wiredcommonly via a liquid crystal cell, the source signal Vs is suppliedfrom the line double-speed conversion part 101 to the source driver 103,and a sample-and-hold voltage of the Vs with a clock CLK supplied fromthe driving control circuit 102 is supplied to a source electrode (S) ofthe TFT 126 through the source lines Y1 to Ym.

Usually, a voltage drop called feedthrough occurs between source anddrain, which generally is corrected on the Vcom side, but theexplanation thereof will be omitted for the purpose of simplification.

Gate signals Vg(n−1), Vg(n), Vg(n+1) respectively are supplied to gatelines Xn−1, Xn, Xn+1, and schematically, it is sufficient for thesesignals to have 2 values of an ON voltage and an OFF voltage.

First, the image signal VI is converted to a double frequency in theline double-speed conversion part 101 based on the synchronous signalSYNC, and this line double-speed image signal is sent to the sourcedriver 103 as the source signal Vs. However, in the present embodiment,instead of sending a double frequency image signal twice, as shown inFIG. 2, a signal for initializing the state of the liquid crystal(hereinafter referred to as an initialization signal, and in the case ofan OCB cell, for example, a black signal of a high level), which is asignal written in the liquid crystal for rendering the OCB liquidcrystal that tries to return to the splay state more likely to remain inthe bend state, is interposed between the double frequency imagesignals, such as in the order of a double frequency image signal, aninitialization signal, a double frequency image signal, aninitialization signal and so on. A voltage supplied to the liquidcrystal cell is a voltage difference between the common voltage Vcom andthe source voltage Vs at a time of gate ON, and an absolute value ofthis voltage difference affects the transmittance of the liquid crystaland the effects of the back transition prevention.

The source signal Vs supplied to each cell is driven in a half cycle ofone horizontal period (1H) such as an image signal, an initializationsignal, an image signal, an initialization signal and so on, and a gateelectrode (G) of the TFT 126 is scanned separately by the gate signal,as shown in FIG. 2, with respect to a timing of switching ON at the timeof an initialization signal and a timing of switching ON at the time ofan image signal. Thus, it is driven as if initialization signals andimage signals are scanned individually.

Therefore, when attention is focused on each cell, it becomes clear thatone frame period (1V) is divided into a back transition preventiondriving period P1 (a first period) and an image signal driving period P2(a second period).

As already described above, it is possible to prevent a back transitionphenomenon of returning to splay configuration by providing aninitialization signal voltage to the cell for a fixed period of time,but the luminance also is deteriorated by writing initializationsignals, so that it is preferable to set this back transition preventiondriving period P1 for writing initialization signals to be as short aspossible.

Generally, a liquid crystal cell in normally white mode has an increasedresponse speed to black when driven by a high voltage. Therefore, whenthe reference voltage to be supplied to the common electrode is changedfrom the conventional 2 kinds to 4 kinds, and when the liquid crystalcell is driven so as to write the Vsup voltage that is a voltage with ahigher absolute value of (Vcom−Vs) when writing initialization signals,rather than when writing image signals, the Vsup hold period serving asthe back transition prevention driving period P1 can be shortened evenmore.

As described above, according to the present embodiment, it becomespossible to adjust the Vsup hold period to an arbitrary length byswitching 4 kinds of the reference voltages Vref1, Vref2, Vref3, Vref4provided to the common driving part 105 and driving the common electrodein synchronization with driving of the source line and the gate line. Asa result, a shortest Vsup hold period capable of suppressing a backtransition can be set easily, and the effects of the screen luminancedeterioration through insertion of a Vsup hold period can be reduced asmuch as possible.

In the experiments conducted by the present inventors, it was confirmedthat a back transition phenomenon can be suppressed when a ratiooccupied by the aforementioned Vsup hold period in one frame period ofan input signal is less than 20%.

Embodiment 2

In Embodiment 1 mentioned above, the Vsup hold period was insertedequally for all the frames. However, a back transition occurs only whena condition continues in which a voltage of a predetermined level orhigher is not applied for longer than a fixed period of time. Therefore,Embodiment 2 of the present invention is configured such that it isjudged whether a signal of a predetermined level or higher is includedin an input signal, and that, only when a signal of a predeterminedlevel or higher is included, a Vsup hold period is set.

Here, the expression “an input signal of a predetermined level orhigher” will be explained. FIG. 17 is a graph showing an appliedvoltage-transmittance curve of a general OCB cell. In FIG. 17, 1701shows an applied voltage-transmittance curve in a case where apredetermined voltage for prevention of a back transition is notinserted, 1702 shows an applied voltage-transmittance curve in a casewhere a predetermined voltage for prevention of a back transition isinserted, and 1703 shows a critical voltage Vth at which a backtransition from bend configuration to splay configuration occurs whenprevention of a back transition is not carried out. When the preventionof a back transition is not carried out, it is returned to splayconfiguration at not more than Vth, so that an appropriate transmittancecan not be obtained, and thus, the OCB cell must be driven at a voltageof not less than Vth, but in this case, a sufficient luminance cannot beobtained. In addition, as shown in FIG. 17, in the case of an OCB cell,its transmittance is reduced as an applied voltage becomes greater,while its transmittance is increased as an applied voltage becomessmaller. In other words, as the level of a displayed image signalbecomes higher, a voltage applied to a pixel is reduced. Therefore, theexpression “an input signal of a predetermined level or higher” isequivalent to the expression “a voltage applied to a pixel cell is of apredetermined level or lower”. This explanation also is applicable tothe embodiments to be described later.

FIG. 3 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 2 of the present invention. InFIG. 3, the liquid crystal display device includes a line double-speedconversion part 101, an input signal level detection part 309, a drivingcontrol part 302, a source driver 103, a gate driver 104, an commondriving part 105, a pixel cell 106, a back light control part 107 and aback light 108.

As is shown in FIG. 3, the liquid crystal display device according toEmbodiment 2 is constructed such that the driving control part 102 inthe liquid crystal display device according to Embodiment 1 mentionedabove is replaced by the driving control part 302, and that the inputsignal level detection part 309 is further added. In addition, theconfiguration of the liquid crystal display device according toEmbodiment 2 other than the above-mentioned elements is the same as thatof the liquid crystal display device according to Embodiment 1 mentionedabove, and the corresponding components have been given the samereference numerals and the explanations thereof are omitted.

In the following, a method of driving a liquid crystal display deviceaccording to Embodiment 2 of the present invention will be described byreferring further to FIG. 4.

FIG. 4 is a timing chart of a control signal for driving the liquidcrystal display device shown in FIG. 3 with respect to a given inputimage signal.

FIG. 4A shows a vertical synchronous signal indicating a frame cycleafter double-speed conversion performed by the line double-speedconversion part 101; FIG. 4B shows an image signal (Vs) afterdouble-speed conversion performed in a like manner; and FIG. 4C shows asignal level detection signal (DS) generated by the input signal leveldetection part 309 in correspondence with a predetermined detectionlevel (A).

The input signal level detection part 309 judges in a frame unit whethera signal of the predetermined level A or higher is included in the inputimage signal Vs and outputs the signal level detection signal DS. Thedriving control part 302 receives this signal level detection signal DS,and when a signal of the predetermined level A or higher is included inthe input image signal, generates a driving control signal for setting aVsup hold period in the next frame. Hereinafter, the same processing asin Embodiment 1 will be performed.

As described above, according to the present embodiment, it is judged ina frame unit whether a signal of a predetermined level or higher isincluded in an input image signal, and when a signal of a predeterminedlevel or higher is included in the input signal, a Vsup hold period isset in the next frame. According to this configuration, it becomespossible to eliminate an unnecessary Vsup hold period and to suppressthe deterioration in the average luminance of a display screen caused bysetting a Vsup hold period.

Embodiment 3

In Embodiment 2 mentioned above, controlling of whether a Vsup holdperiod is to be set or not was performed in correspondence with thelevel of an input image signal. At this time, the average luminance of adisplay screen changes between frames in which a Vsup hold period is setand a Vsup hold period is not set. Therefore, Embodiment 3 of thepresent invention is configured to reduce this sense of visualincongruity caused by the change in luminance.

FIG. 5 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 3 of the present invention. InFIG. 5, the liquid crystal display device includes a line double-speedconversion part 101, an input signal level detection part 509, a drivingcontrol part 502, a source driver 103, a gate driver 104, an commondriving part 105, a pixel cell 106, a back light control part 107 and aback light 108.

As is shown in FIG. 5, the liquid crystal display device according toEmbodiment 3 is constructed such that the driving control part 302 inthe liquid crystal display device according to Embodiment 2 mentionedabove is replaced by the driving control part 502, and that the inputsignal level detection part 309 is replaced by the input signal leveldetection part 509. In addition, the configuration of the liquid crystaldisplay device according to Embodiment 3 other than the above-mentionedelements is the same as that of the liquid crystal display deviceaccording to Embodiment 2 mentioned above, and the correspondingcomponents have been given the same reference numerals and theexplanations thereof are omitted.

In the following, a driving method of a liquid crystal display deviceaccording to Embodiment 3 of the present invention will be described byreferring further to FIG. 6.

FIG. 6 is a timing chart of a control signal for driving the liquidcrystal display device with respect to a given input image signal.

FIG. 6A shows a vertical synchronous signal indicating a frame cycleafter double-speed conversion performed by the line double-speedconversion part 101; FIG. 6B shows an image signal (Vs) afterdouble-speed conversion performed in a like manner; and FIG. 6C shows aVsup period provision signal generated by the input signal leveldetection part 509 in correspondence with a predetermined detectionlevel A.

The input signal level detection part 509 judges in a frame unit whethera signal of the predetermined level A or higher is included in the inputimage signal Vs and outputs the Vsup period provision signal (Vsup PS).The Vsup period provision signal VsupPS is a signal that defines thelength of a Vsup period in that particular frame. In FIG. 6, this signalis shown as having a switching precision of 5 stages in length.

When a signal of the predetermined level A or higher is included in theinput image signal of the current frame, the input signal leveldetection part 509 generates a Vsup period provision signal VsupPS thatsets the length of a Vsup period in the next frame to be longer by 1stage at most than the length of a Vsup period in the current frame.Furthermore, when a signal of the predetermined level or higher is notincluded in the input image signal of the current frame, the inputsignal level detection part 509 generates a Vsup period provision signalVsupPS that sets the length of a Vsup period in the next frame to beshorter by 1 stage at most.

The driving control part 502 receives this Vsup period provision signalVsupPS and generates a control signal so that a Vsup period incorrespondence with this value is set.

As described above, according to the present embodiment, it is judged inframe unit whether a signal of a predetermined level or higher isincluded in an input image signal, and the length of a Vsup period ischanged continuously between frames based on this result of judgement.According to this configuration, it becomes possible to eliminateunnecessary Vsup hold periods, while suppressing the change in averageluminance of a display screen caused by the change in the Vsup periodbetween the frames, and the deterioration in the average luminance of adisplay screen caused by setting a Vsup hold period can be suppressed.

In addition, FIG. 6C shows that the Vsup period provision signal Vsup PScan be selected at 5 stages, but if more gradation is given, a change inluminance caused by the difference in length of the Vsup period betweenthe frames can be suppressed to that extent, so that it is preferable.Moreover, the relationship between the gradation of the Vsup periodprovision signal VsupPS and the actual length of the Vsup periodcorresponding thereto may be linear, or a nonlinear type that isdetermined depending on the gradation of the Vsup period provisionsignal Vsup PS in the current frame.

Embodiment 4

In Embodiment 3 mentioned above, controlling of changing the length of aVsup hold period between frames was performed continuously incorrespondence with the level of an input image signal. At this time,along with the change in length of the Vsup hold period, an averageluminance of a display screen changes while admitting that it is acontinuous change. Embodiment 4 of the present invention is configuredto suppress this change in luminance.

FIG. 7 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 4 of the present invention. InFIG. 7, the liquid crystal display device includes a line double-speedconversion part 101, an input signal level detection part 509, a drivingcontrol part 502, a source driver 103, a gate driver 104, an commondriving part 105, a pixel cell 106, a back light control part 707, and aback light 108.

As is shown in FIG. 7, the liquid crystal display device according toEmbodiment 4 is constructed such that the back light control part 707 isnewly added to the liquid crystal display device according to Embodiment3 mentioned above. In addition, the configuration of the liquid crystaldisplay device according to Embodiment 4 other than the above-mentionedelement is the same as that of the liquid crystal display deviceaccording to Embodiment 3 mentioned above, and the correspondingcomponents have been given the same reference numerals and theexplanations thereof are omitted. Moreover, a method for controlling aback light luminance is a process performed conventionally, so that adetailed explanation thereof is omitted here.

In the following, a driving method of a liquid crystal display deviceaccording to Embodiment 4 of the present invention will be described byreferring further to FIG. 8.

FIG. 8 is a timing chart of a control signal for driving the liquidcrystal display device with respect to a given input image signal.

A vertical synchronous signal indicating a frame cycle afterdouble-speed conversion performed by the line double-speed conversionpart 101, an image signal (Vs) after double-speed conversion performedin a like manner and a Vsup period provision signal VsupPS generated bythe input signal level detection part 509 in correspondence with apredetermined detection level A shown respectively in FIG. 8A, FIG. 8Band FIG. 8C are all identical to the signals in Embodiment 3.

As shown in FIG. 8D, the back light control part 707 receives the Vsupperiod provision signal VsupPS from the input signal level detectionpart 509 and generates a back light luminance control signal BC′ forlighting the back light 108 with a luminosity that compensates for achange in average luminance of a display screen caused by a change inthe Vsup period determined by the Vsup period provision signal VsupPS.

As is clear from FIG. 8, the back light luminance control signal BC′ isdetermined in correspondence with the Vsup period provision signalVsupPS and controls the back light 108 such that the back lightluminance becomes brighter when the Vsup period is long, that is, theaverage luminance of the display screen is reduced, and to the contrary,the back light luminance becomes darker when the Vsup period is short,that is, the average luminance of the display screen is raised.

As described above, according to the present embodiment, the length of aVsup period and the brightness of a back light are controlled inconjunction with each other, so that it becomes possible to suppress achange in the average luminance of a display screen caused by thepresence or absence of a Vsup period, while maintaining the suppressioneffects of a back transition.

Embodiment 5

As already described, a back transition occurs when a conditioncontinues in which a voltage of a predetermined level or higher is notapplied to a certain pixel for longer than a fixed period of time, andcontrolling of setting a Vsup hold period when a signal of apredetermined level A or higher is included in one frame of an inputimage signal as performed in Embodiments 2 to 4 satisfies sufficientconditions for suppressing a back transition. Embodiment 5 of thepresent invention is configured to conduct a more detailed determinationof the frames that require setting of a Vsup period.

FIG. 9 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 5 of the present invention. InFIG. 9, the liquid crystal display device includes a line double-speedconversion part 101, a driving control part 902, a source driver 103, agate driver 104, an common driving part 105, a pixel cell 106, a backlight control part 107, a back light 108 and an input signal movementdetection part 909.

As is shown in FIG. 9, the liquid crystal display device according toEmbodiment 5 is constructed such that the driving control part 502 inthe liquid crystal display device according to Embodiment 3 mentionedabove is replaced by the driving control part 902, and that the inputsignal movement detection part 909 is newly added. In addition, theconfiguration of the liquid crystal display device according toEmbodiment 5 other than the above-mentioned elements is the same as thatof the liquid crystal display device according to Embodiment 2 mentionedabove, and the corresponding components have been given the samereference numerals and the explanation thereof is omitted.

The input signal movement detection part 909 receives an image signal(VI) and a synchronous signal (SYNC) to be input to the liquid crystaldisplay device and judges whether the input image signal is a movingimage or a static image. The input signal movement detection part 909has a memory capable of retaining an image signal for one frame,compares an image signal in the immediately preceding frame with thecurrent input image signal written in each pixel, and judges inconsideration also of the effects of noise included in the image signalthat a pixel having this difference of a predetermined level or lower isa pixel without movement with respect to the preceding frame(hereinafter referred to as a static pixel). When a number of thisstatic pixel is not more than a predetermined number, the input imagesignal is judged as a moving image. However, when there is a staticpixel of a predetermined level or lower in a number of not less than apredetermined number, the input image signal is judged as a staticimage. The input signal movement detection part 909 outputs theabove-mentioned result of judgement as a movement detection signal (MD)to the driving control part 902. The driving control part 902 sets aVsup period only when the input image signal is judged as a static imageby the input signal movement detection part 909 based on the movementdetection signal MD.

As described above, according to the present embodiment, when there is alarge number of pixels in which a difference between the image signalretained in the immediately preceding frame and the image signal in thecurrent frame is of a predetermined level or lower, the input image isjudged as a static image and a Vsup period is set. On the other hand, asfor frames in which no possibility of a back transition exists, a Vsupperiod is not set. Thus, unnecessary deterioration in the averageluminance of a display screen can be suppressed.

In addition, the present embodiment was described in that a Vsup periodis set only when the input image signal is judged as a static image bythe input signal movement detection part 909. However, it may becontrolled so as to elongate the Vsup period when it is judged as astatic image and to shorten the Vsup period when it is judged as amoving image. Alternatively, it may be controlled so as to raise thevoltage of Vsup when it is judged as a static image and to lower thevoltage of Vsup when it is judged as a moving image. Furthermore, thejudgement on whether it is a static image or a moving image was made byusing only data in one frame, but this judgement may be made by usingdata in a plurality of frames.

Moreover, an image signal and a synchronous signal that were input tothe liquid crystal display device were used as an input for the inputsignal movement detection part 909, but a line double-speed image signal(Vs) and a synchronous signal output from the line double-speedconversion part 101 may be used.

Embodiment 6

It has become clear from the evaluations made by the present inventorsthat the suppression effects of a back transition are high when the Vsupperiod is elongated although the average luminance of a display screenis deteriorated. Therefore, Embodiment 6 of the present invention isconfigured to perform controlling by setting a comparatively long Vsupperiod based on the judgement that even if the Vsup period is elongatedat the boundary of frames where the characteristics of an input imageare changed greatly compared to frames before and after thereof, thevisual effects resulting from a change in average luminance thereby aresmall.

FIG. 10 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 6 of the present invention. InFIG. 10, the liquid crystal display device includes a line double-speedconversion part 101, a driving control part 1002, a source driver 103, agate driver 104, an common driving part 105, a pixel cell 106, a backlight control part 107, a back light 108 and a scene change detectionpart 1009.

As is shown in FIG. 10, the liquid crystal display device according toEmbodiment 6 is constructed such that the driving control part 502 inthe liquid crystal display device according to Embodiment 3 mentionedabove is replaced by the driving control part 1002, and that the scenechange detection part 1009 is newly added. In addition, theconfiguration of the liquid crystal display device according toEmbodiment 6 other than the above-mentioned elements is the same as thatof the liquid crystal display device according to Embodiment 2 mentionedabove, and the corresponding components have been given the samereference numerals and the explanation thereof is omitted.

The scene change detection part 1009 is a part for detecting a change inthe characteristics of an input image signal when the change is great.In the experiments conducted by the present inventors, it was confirmedthat detection precision can be obtained to some degree by the followingcomparatively simple configuration.

In other words, detection is performed by focusing attention on theaverage luminance level of display data in each frame (hereinafterreferred to as an APL). The detection of an APL is a process performedconventionally, so that a detailed explanation thereof is omitted here.The scene change detection part 1009 receives an image signal that isinput to the present liquid crystal display device to retain an APL inthe immediately preceding frame (hereinafter referred to as an APLpre)and to calculate an APL in this frame at the end of one frame(hereinafter referred to as an APLnow). Then, when the absolute value ofthe difference between the APLpre and the APLnow is of a predeterminedlevel or higher, it is judged that the characteristics of the image havechanged greatly.

When it is judged that the characteristics of the image have changedgreatly, the scene change detection part 1009 renders a scene changedetection signal (SCD) to be in an active state during the next oneframe. In the frame in which the scene change detection signal SCD is inthe active state, the driving control part 1002 sets the Vsup holdperiod to be longer by a predetermined time than other frames.

As described above, according to the present embodiment, by setting theVsup period to become longer at the boundary of the frames where thecharacteristics of the input image are changed greatly, it becomespossible to enhance the suppression effects of a back transition, whilesuppressing the visual effects resulting from the change in averageluminance of a display screen caused by the difference in the length ofthe Vsup period.

In addition, in the present embodiment, the scene change detection part1009 was described as a part that calculates the APL in one frame as awhole and detects the characteristic change of the image by using thisAPL. However, the detection precision of the characteristic change canbe improved by dividing one screen into a plurality of regions,calculating an APL for each region, and comparing this plurality of APLbetween the frames respectively.

Furthermore, it may be controlled such that the voltage of Vsup is setto be higher in the frame in which the scene change detection signal SCDis in the active state than in other frames.

Moreover, an input image signal and a synchronous signal were used as aninput for the scene change detection part 1009, but a line double-speedimage signal (Vs) and a synchronous signal output from the linedouble-speed conversion part 101 may be used.

Embodiment 7

In Embodiments 1 to 6 mentioned above, the source driver 103 was driventwice as fast by doubling the speed. In other words, twice as muchtransfer speed as an ordinary transfer speed was necessary, and when apanel with a large number of pixels is driven, higher performance isrequired for transfer and signal processing.

In fact, the panel with a large number of pixels generally is designedto reduce its transfer speed by using the conventional techniques ofdoubling a transfer bus width to the source driver 103 or the like, andit is not desirable for the panel with a large number of pixels tosimply double the clock.

Embodiment 7 of the present invention is configured to set a Vsupperiod, while maintaining this transfer speed as conventionally.

FIG. 11 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 7 of the present invention. InFIG. 11, the liquid crystal display device includes a line double-speedconversion part 101, a driving control part 1102, a reference voltageswitching part 1110, a source driver 1103, a gate driver 104, an commondriving part 105, a pixel cell 106, a back light control part 107 and aback light 108.

As shown in FIG. 11, digital data Vs that were input to the sourcedriver 1103 are transferred to an internal shift register including aplurality of flip-flops 1103-1, and after a transfer for all the pixelsis completed, a D/A enable signal (D/AEN) from the driving control part1102 renders the D/A conversion part 1103-2 to be in an enable state.Furthermore, the driving control part 1102 outputs a switching signal(SP) to the reference voltage switching part 1110, and the referencevoltage switching part 1110 selects a reference voltage VREF1 (voltage(A): black level–voltage (E): white level) serving as the input-outputcharacteristics shown in FIG. 12A and outputs it to the D/A conversionpart 1103-2. Based on the reference voltage VREF1 that was input fromthe reference voltage switching part 1110, the D/A conversion part1103-2 converts it into source voltages Y1 to Ym for each pixel in whichthe input digital data for each pixel were subject to the gammacorrection shown in FIG. 12A.

Furthermore, by switching the reference voltage VREF to a referencevoltage VREF2 (voltage (A): white level–voltage (E): black level) so asto realize completely reversed input-output characteristics of FIG. 12Aas shown in FIG. 12B, the so-called source inversion can be achieved.

Furthermore, by switching the reference voltage VREF to a referencevoltage VREF 3 (voltage (A)–voltage (E): black level) for aninitialization signal in the initialization signal write period as shownin FIG. 12C, the voltage after the D/A conversion becomes aninitialization signal regardless of whatever data are transferred to theshift register inside the source driver 1103.

In addition, by switching the reference voltage VREF to a referencevoltage VREF 4 (voltage (A)–voltage (E): black level) for an invertedinitialization signal in the initialization signal write period as shownin FIG. 12D, the voltage after the D/A conversion becomes an invertedinitialization signal regardless of whatever data are transferred to theshift register inside the source driver 1103.

As described above, according to the present embodiment, it is no longernecessary to transfer an initialization signal, and thus, both the imagesignal and the initialization signal can be driven to a liquid crystalcell, while maintaining the transfer speed of the source driver asconventionally.

Embodiment 8

As is disclosed, for example, in JP9(1998)-325715A, it is widely knownwith respect to a hold type display element such as a liquid crystalthat human visual integral characteristics deteriorate the visibility ofmoving images.

Therefore, Embodiment 8 of the present invention is configured toprovide a liquid crystal display device in which a back transition isprevented by changing a Vsup hold period depending on whether the imageis a moving image or a static image, while luminance deterioration issuppressed to be a minimum in the case of a static image and thevisibility of moving images is improved in the case of a moving image.

FIG. 13 is a block diagram showing the configuration of a liquid crystaldisplay device according to Embodiment 8 of the present invention. InFIG. 13, the liquid crystal display device includes a line double-speedconversion part 101, a driving control part 1302, a source driver 103, agate driver 104, an common driving part 105, a pixel cell 106, a backlight control part 107, a back light 108 and an input signal movementdetection part 909.

As shown in FIG. 13, it is detected in the input signal movementdetection part 909 whether the input image signal is a moving image or astatic image, and the driving control circuit 1302 generates a controlsignal such that the Vsup hold period is set longer when it is judged asa moving image, and that the Vsup hold period is set shorter when it isjudged as a static image.

In addition, Embodiment 5 mentioned above is configured only for thepurpose of preventing a reverse phenomenon from occurring. Therefore, itwas controlled not to set the Vsup hold period or to set it shorter whenit was judged that the input image signal is a moving image and to setthe Vsup hold period longer when it was judged that the input imagesignal is a static image. However, the present embodiment is configuredto perform the controlling as mentioned above both for the purpose ofpreventing a reverse phenomenon from occurring and improving thevisibility of moving images.

As described above, according to the present embodiment, while a backtransition is prevented by changing the Vsup hold period depending onwhether the input image signal is a moving image or a static image, theluminance deterioration is suppressed to be a minimum in the case of astatic image, and the visibility of moving images can be improved in thecase of a moving image by elongating the period of an initializationsignal and thus approximating it to impulse type display driving such asCRT.

As described above, according to the present invention, it becomespossible to easily set the shortest Vsup hold period and the minimumVsup voltage suppressing an occurrence of a back transition and capableof suppressing a back transition, and to display excellent images byreducing the effects of luminance deterioration on a screen as much aspossible by inserting a Vsup hold period.

1. A method for driving a liquid crystal display device having a liquidcrystal panel, the liquid crystal panel comprising a plurality of sourcelines to which pixel data are supplied, a plurality of gate lines towhich scanning signals are supplied, pixel cells positioned in matrixform in correspondence with intersecting points of the source lines andthe gate lines, a source driver that drives the source lines based on aninput image signal, a gate driver that drives the gate lines, and a backlight, the pixel cells being applied with a signal for initializing astate of a liquid crystal therein as well as pixel data incorrespondence with the image signal in the pixel cells, wherein a firstperiod for writing the signal for initializing a state of the pixelcells and a second period for writing the pixel data are provided in oneframe period, a length of the first period is set variably for eachframe, a voltage level to be applied to each pixel cell in the firstperiod is set such that each pixel cells retains a voltage Vsup higherthan a voltage level to be applied to each pixel cell in the secondperiod, and the length of the first period is controlled by a result ofcalculating an average luminance level by an image signal input in apredetermined number of preceding frames and an average luminance levelby an image signal to be input in a current frame.
 2. The method fordriving a liquid crystal display device according to claim 1, whereinwhen a difference between an average luminance level by an image signalinput in a predetermined number of preceding frames and an averageluminance level by an image signal to be input in a current frame islarger that a predetermined level, the first period is set in apredetermined length in a next frame.
 3. A method for driving a liquidcrystal display device having a liquid crystal panel, the liquid crystalpanel comprising a plurality of source lines to which pixel data aresupplied, a plurality of gate lines to which scanning signals aresupplied, pixel cells positioned in matrix form in correspondence withintersecting points of the source lines and the gate lines, a sourcedriver that drives the source lines based on an input image signal, agate driver that drives the gate lines, and a back light, the pixelcells being applied with a signal for initializing a state of a liquidcrystal therein as well as pixel data in correspondence with the imagesignal in the pixel cells, wherein a first period for writing the signalfor initializing a state of the pixel cells and a second period forwriting the pixel data are provided in one frame period, a length of thefirst period is set variably for each frame, a voltage level to beapplied to each pixel cell in the first period is set such that eachpixel cells retains a voltage Vsup higher than a voltage level to beapplied to each pixel cell in the second period, and it is detectedwhether an input image signal is a moving image or a static image, andas a result of detection, the first period is set longer than apredetermined length when it is judged that the input image signal is amoving image, and the first period is set shorter than a predeterminedlength when it is judged that the input image signal is a static image.